1. Field of the Invention
The present invention relates to a chopper amplifier circuit and a semiconductor device having the chopper amplifier circuit.
2. Description of the Related Arts
FIG. 2 is a block diagram showing a conventional chopper amplifier circuit. In the conventional chopper amplifier circuit 20, multipliers 11 and 12 are provided in front and back stages of an amplifier circuit 1, respectively. The multipliers 11 and 12 are controlled with a modulation signal CLK in a square wave with a frequency fc. The output of the chopper amplifier circuit 20 is connected to a low-pass filter 22 (see P. Allen and D. R. Holberg, CMOS Analog Circuit Design, P490-494, Saunders College Publishing, 1987).
FIGS. 3A to 3F show frequency characteristics of an input signal in each portion of the conventional chopper amplifier circuit. It is assumed that an input signal has frequency characteristics as shown in FIG. 3A at input terminals 5 of the chopper amplifier circuit 20. Further, it is assumed that the amplifier circuit 1 has an input conversion noise and an offset voltage Vn of the frequency characteristics as shown in FIG. 3C. After the input signal passes through the multiplier 11, the input signal is modulated to a frequency that is an odd multiple of the frequency fc of the modulation signal CLK. FIG. 3B shows frequency characteristics of the input signal at this time. The modulated input signal is amplified and output after being supplied with the input conversion noise and the offset voltage Vn of the amplifier circuit 1. FIG. 3D shows frequency characteristics of the input signal at the output of the amplifier circuit 1. The input signal output from the amplifier circuit 1 is modulated to an original frequency band (i.e., a low-frequency region including a DC) through the multiplier 12. On the other hand, the noise component and the offset voltage Vn at the input of the amplifier circuit 1 are modulated to a frequency of an odd multiple of the frequency fc of the modulation signal CLK. FIG. 3E shows frequency characteristics of an input signal at the output of the multiplier 12. Further, the input signal output from the multiplier 12 passes through the low-pass filter 22, whereby a high frequency component of the modulation signal CLK is removed. Thus, as shown in FIG. 3F, only an input signal component can be amplified without amplifying the noise and offset voltage of the amplifier circuit 1.
Further, in another conventional chopper amplifier circuit, an input signal is subjected to double chopper modulation using modulation signals with two different frequencies, whereby the noise and the offset voltage of an amplifier circuit used in a chopper amplifier circuit are further reduced (see U.S. Pat. No. 6,262,626, Bakker, et al., Jul. 17, 2001).
However, the conventional chopper amplifier circuit has the following problems. When the conventional chopper amplifier circuit is used, for example, for amplifying the output voltage of a sensor bridge using a piezoresistor, the offset voltage of the sensor bridge cannot be cancelled due to incomplete matching of the piezoresistor. Therefore, the offset voltage of the sensor bridge is amplified to be output.
Further, the offset voltage of the sensor bridge using the piezoresistor has temperature characteristics, and the temperature characteristics of the offset voltage appear in the output voltage of the chopper amplifier circuit.